Ultra-pure magnesium fluoride (MgF.sub.2) has been made by a variety of laboratory methods with no regard for the pressability of the MgF.sub.2 powder. Purity of MgF.sub.2 powder is not determinative of its pressability, and only that powder is pressable which consists of deformable particles. The commercial preparation of the ultra-pure alkali metal fluorides and alkaline earth metal fluorides is of relatively recent interest, this interest having been sparked by the widespread use of these fluoride powders for hot-pressed infra-red radiation transmitting windows, and other optical elements which are now commonplace.
It is to be noted that a pressable powder consists of "deformable" individual particles, and, conventional powder unless specified otherwise, does not. Deformable particles are defined as those which may be physically deformed under sufficiently high heat and pressure conditions to form an optically integral body. Acceptable infra-red radiation transmitting bodies of inorganic crystalline ionic halides are generally characterized by relatively high transmittance of infra-red radiation, often in excess of 90%, over a relatively wide range of wavelengths up to about 10.mu. (microns), depending upon the particular crystalline halide used. Magnesium fluoride optics are of particular importance for transmittance in the range from about 1.mu. to about 7.mu.. In this relatively wide range, it is difficult to hot-press even essentially pure magnesium fluoride powder, irrespective of how it is obtained, into a hot-pressed optical body essentially free of absorption bands. That high purity is not the essential criterion of pressability is evident from the fact that high purity crystals obtained by grinding a melt-grown ingot of MgF.sub.2 are not pressable into an optical body. Neither are crystalline powders obtained from hot-pressed MgF.sub. 2 optical bodies. This difficulty is attributable to the physical characteristics of the unpressable particles, such as crystalline structure, size and shape, and also to their purity which characteristics preclude deformability under hot-pressing conditions. Throughout this specification the term "pressable" is used to describe a mass of deformable particles in the strict sense that it connotes that the mass can be hot-pressed into an optically integral body which will not scatter radiation in a predetermined wavelength. There is a vast difference between particles which yield a solid body which may be essentially opaque, and an optically integral body. Though it is difficult, if not impossible, to define a pressable powder in absolute terms the distinction between a pressable powder and one that is not, is immediately apparent after each is pressed under sufficient pressure and at a temperature below the melting point.
It is well known that even high-quality hot-pressed magnesium fluoride optical bodies are not uniformly permeable to radiation throughout all regions of a transmittance spectrum, but exhibit absorption bands therewithin which are undesirable in many applications. Absorption bands corresponding to hydroxyl (OH.sup.-) at about 2.8.mu., to carbon dioxide (CO.sub.2) at about 4.3.mu., to bifluorides (HF.sub.2.sup.-) at 5.0.mu., and to carbonate (CO.sub.3.sup.=) at 7.0.mu. wavelengths, as well as the absorption bands at 3.0.mu. and 6.1.mu. for moisture, inter alia, are particularly troublesome in the development of high-quality infra-red wavelength transmitting elements made of magnesium fluoride.
Absorption bands in hot-pressed magnesium fluoride were recognized early in the development of optical bodies from hot-pressed powder. For example, a typical transmittance curve (See "Hot Pressing Magnesium Fluoride" by Buckner, Hafner and Kreidl, Jour. of Amer. Ceramic Soc. 34, No. 9, pages 425-438, 1962) for a hot pressed magnesium fluoride optical body shows several sharp absorption bands which occur at the frequency usually taken as --O--H bending. It is to the minimization of these impurities with economy, that the instant invention is concerned.
More specifically, no presently available magnesium fluoride powder may be hot-pressed in the range from about 15,000 psi to about 80,000 psi, to yield an optical body which is simultaneously free of absorption bands due to CO.sub.2, H.sub.2 O, carbonate and hydroxyl ions, and acid fluorides (thought to be mainly HF.sub.2.sup.- and homologous ions of a similar nature). Furthermore, no presently available pressable MgF.sub.2 powder is available in a particle size range which may be directly hot-pressed, without grinding, or, which has a specific, uniform refractive index n.sub.D = 1.3850 at 20.degree. C, for all particles. It is to meet, not only the increasingly stringent quality specifications demanded of hot-pressed magnesium fluoride optical bodies, but also a demand for a powder with predictably consistent hot-pressing characteristics that the instant invention is directed.